Traxxas 3951X 21-tooth heavy-duty pinion gear is machined from hardened steel and fits 3mm shafts. Includes set screw. Use Traxxas stock and hop-up replacement parts to get the most out of your Traxxas RTR vehicles. This product is made from high quality materials and designed for long-lasting performance. This is replacement hardware for use on Traxxas vehicles; consult your user's manual for exact parts listings.